1. Field of the Invention
The present invention relates to an optical node having a plurality of receiving ports receiving wavelength division multiplex optical signals which require Raman amplification for low noise characteristic, and more particularly an optical node capable of controlling Raman power distribution easily even in case of extension or deletion of the receiving ports.
2. Description of the Related Art
In recent years, a photonic network employing the wavelength division multiplex (WDM) optical transmission technique has been constructed for practical application. In addition to point-to-point transmission having been introduced so far, optical nodes for connecting more than two points and distributing multiplexed signals to a plurality of ports (hereafter the term ‘node’ conceptually includes optical cross connect equipment, optical add/drop multiplexer, etc.) are disposed in the photonic network shown in FIG. 1. With such two-dimensional deployment of the optical transmission (to a mesh network), efficient allocation of network apparatuses is being promoted.
Here, in a backbone network, it has been a known technique that a plurality of ports are concentrated in one point, and signals are distributed to the different ports in the form of light, and thereby network flexibility is increased with reduced network costs. For example, in FIG. 1, in anode #2, optical signals are received from nodes #0 and #1, and distributed to the ports toward nodes #3 and #4.
When such a multi-port distribution node in which signals on a plurality of receiving ports are multiplexed and distributed to one or a plurality of transmitting ports is applied, when transmission spans 101, 102, 103, 104 of the wavelength or channel paths passing through these nodes are long, low noise is required in these channel paths.
To meet the above requirement, in the conventional example, there has been incorporated a method of disposing Raman fiber amplifiers for Raman amplification, in which excitation light is input to each receiving node side of the long-haul optical transmission ports.
FIG. 2 shows an explanation diagram illustrating the principle of the Raman fiber amplifier. Optical fiber Raman amplification (simply, Raman amplification) is a phenomenon of stimulated emission being produced by Raman scattering when strong excitation light is incident on an optical fiber, and amplification being obtained on a wavelength region approximately 100 nm longer than the optical excitation wavelength.
Accordingly, for example, in case of multiplexing three channels, excitation light fp1, fp2, fp3 each corresponding to the three cannels is transmitted from a Raman light source LD to each receiving line side. As a result, an amplification function is performed on each wavelength range fr1, fr2, fr3, which is approximately 100 nm longer than the corresponding optical excitation wavelength, and flatness property of the wavelength amplification can be obtained in an optical signal range.
In FIG. 1, in order to perform Raman amplification in optical transmission line spans 101, 102, the excitation light from a Raman light source LD is forwarded on two receiving port sides from node #2. Similarly, in order to perform Raman amplification in optical transmission line spans 103, 104, the excitation light from each Raman light source LD in node #3, #4 is forwarded on each receiving port side.
Here, such a Raman fiber amplification apparatus requires large power consumption, size and cost. Therefore, it requires increased power consumption, size and cost to distribute the excitation light to the plurality of receiving ports in each node. This may impede installation of such a node.
FIG. 3 shows an exemplary configuration of a multi-port distribution optical node, having a Raman optical amplifier disposed on each receiving port side.
Corresponding to each transmission line fiber 10, a pump light source 3 for distributed Raman amplification is provided. In a wavelength division multiplex (WDM) coupler 2, signal light 1 is multiplexed with pump light from pump light source 3. The output of WDM coupler 2 is input to an optical multiplex & distribution unit 5 having an optical cross connect function or an optical add/drop function, through an EDFA (Erbium-doped fiber amplifier) 4.
In FIG. 3, the amplified signal light 1 is transmitted on an output terminal. Meanwhile, pump light is supplied from pump light source 3 for Raman amplification, to transmission line fiber 10 through WDM coupler 2. With this, signal light 1 is Raman amplified in transmission line fiber 10, amplified in EDFA 4, and divided on a wavelength-by-wavelength basis in optical multiplex & distribution unit 5, and then distributed to each corresponding port. Thereafter, signal light 1 is amplified in EDFA 41, which is disposed on each output port side, and forwarded to each transmission line fiber 10.
Such disposition of the costly Raman amplification pump 4 on each receiving port results in a high optical node cost. This becomes an impeding factor against optical node application.
As a second conventional example, FIG. 4 shows an optical node configuration having a pump light source for distributed Raman amplification concentrated into one, which has been disclosed in the official gazette of the Japanese Unexamined Patent Publication No. 2002-6349. As shown in FIG. 4, in optical multiplex & distribution unit 5, a Raman pump common supply unit 30 is commonly provided for a plurality of transmission line fibers 10.
Raman pump common supply unit 30 includes a plurality of pump light sources 3 for Raman amplification, each provided for each of a plurality of wavelengths or channels; and a coupler 6 for synthesizing the outputs of pump light sources 31, 32, 33. The outputs of pump light sources 31, 32, 33 for Raman amplification each provided for each of the plurality of wavelengths are controlled by an LD controller 7.
More specifically, the pump light from pump light source 3 for Raman amplification is supplied to transmission line fibers 101–104 via pump coupler 6 and WDM couplers 21–24, and Raman amplified. The Raman-amplified signal light 1 is further amplified in EDFA 41–44, and divided wavelength by wavelength in optical multiplex & distribution unit 5, and further distributed and output to each predetermined port. Further, signal light 1 is amplified on the output port side by EDFA 45–48, and supplied to transmission line fibers 105–108.
According to the configuration shown in this FIG. 4, Raman pump common supply unit 30 is provided. However, the total pump power is not decreased, as compared with the configuration shown in FIG. 3. In addition, because the power distributed to each port is fixed by a branching ratio of pump coupler 6, there is a problem of lack of flexibility in power distribution, which is to be secured at the time of extension or deletion of ports.
Meanwhile, in the distributed Raman amplification which feeds excitation light to be transmitted to the transmission line in the reverse direction to the signal light, it has been known that a response time when the excitation light functions to a signal light gain is delayed to several hundred microseconds. By utilizing this phenomenon, a method has been proposed in the U.S. Pat. No. 6,611,368, and in a conference paper “Tuning speed requirements for time-division multiplexed Raman pump lasers” by Winzer et al., 4.1.4, ECOC (European Conference on Optical Communication) 2002. According to the above disclosure, the excitation light of a plurality of different wavelengths or channels is time-division multiplexed at a frequency sufficiently higher than the Raman response speed.
Using this method, depending on a condition, it becomes possible to improve noise characteristic by approximately 1 dB.
In FIG. 5, an explanation diagram illustrating the principle of this method of time-division multiplexing of the Raman excitation light is shown. In this FIG. 5, Raman light sources 31, 32, 33 outputting Raman excitation light having frequencies λ1, λ2, λ3 emit light at all times. The output light is input to each corresponding optical modulator 81, 82, 83, which is formed of ferroelectric crystal such as LiNbO3 and LiTaO3.
Optical modulators 81, 82, 83 performs pulse modulation using time-division control pulses having mutually shifted phases fed from a pulse generator 80. The outputs of optical modulators 81, 82, 83 are coupled by an optical coupler 6, and further transferred, via a coupler 2, to an optical transmission line 10 in the reverse direction to the direction of optical signal transmission.
Accordingly, on optical transmission line 10, as shown in the enclosed chart shown in the lower left of FIG. 5, Raman excitation light having wavelengths λ1, λ2, λ3 is time-division multiplexed and forwarded at a period not greater than 0.1 msec, namely at a speed not lower than 10 kHz. By this, a Raman amplified wavelength-multiplexed optical signal having wavelengths λ1, λ2, λ3 appears on the output of coupler 2, which is further amplified in EDFA 4.
According to the above paper “Tuning speed requirements for time-division multiplexed Raman pump lasers”, ECOC2002, the method shown in FIG. 5 is reported effective to prevent deterioration of gain efficiency, and a noise characteristic, ordinarily produced between non-modulated excitation light by the Raman effect, which is ordinarily produced on the signal light on the shorter wavelength side. A modulation frequency required in a system shown by the above paper is normally no less than 10 kHz. However, the pump power is not utilized efficiently, since a light extinction time, that is, a time the Raman excitation light is not emitted is long (as shown in FIG. 5 by the bold line in the pulse period generated by pulse generator 80).
To cope with this problem, as another method, it is possible to consider about more effective use of the pump power, which is attained by distributing the pump light even at the above-mentioned light extinction time to the other ports on a time division basis.